Look for something that will deliver the volume (SPL) you want. For a compute speaker, you don't need that much volume if you use it close to you (say less than 2 ft from your head). But as a dock to fill an entire room, you'll want more max SPL.

Subwoofer? I thought you said it was a "small" project? But seriously, if you want a sub that does well, it won't be small but it doesn't have to be hard to make. If it's intended to supplement your full-range computer speaker, you can finish the full-range first, then build the sub to integrate with the full-range speakers. The only concern I would have is that a sub is normally goes no higher than say 100 Hz on the high end (most home theater specs such as DD are 80Hz for low pass) so your full-range need to go down into that range to integrate with the sub properly.